Sustainable Energy, 2014, Vol. 2, No. 1, 12-19 Available online at http://pubs.sciepub.com/rse/2/1/3 © Science and Education Publishing DOI:10.12691/rse-2-1-3

Biogas Production Using Geomembrane Plastic Digesters as Alternative Rural Energy Source and Soil Fertility Management

Seid Yimer1, Bezabih Yimer2, Omprakash Sahu1,*

1Department of Chemical Engineering, KIOT, Wollo University, Kombolcha, Ethiopia 2Mersa Agricultural TVET College, Woldya, Ethiopia *Corresponding author: [email protected] Received December 18, 2013; Revised January 10, 2014; Accepted January 20, 2014 Abstract The aim of work is to evaluate amount of gas production, economical feasibility and quality of slurry for geomembrane plastic biogas plants constructed below and above the ground surface and fixed-dome biogas plant of 3 m3 capacity. Amount of gas and slurry were measured using calibrated biogas burner and weight balance respectively. The qualities of the slurry were analyzed in the laboratory using Kjeldahl and ash method respectively. Economic evaluation and comparison of the biodigester was carried out using cost-benefit analysis. Gas production and total-N was higher for a single layered and above ground plastic biodigester than others. Fermented slurry contained larger nitrogen content than fresh cow dung in both models of biodigester. The geomembrane plastic biogas plant gave higher net benefit than fixed-dome biogas plant. The biogas technology was found to increase income generation through increased crop production with the use of nutritive slurry as organic fertilizer and solve the problem of fuel shortage in the rural areas. Environmental impact assessment of the technology was studied and 3 3 found that from the use of a geomembrane plastic biodigester, 360 m of CO2 and 600 m CH4 was prevented from emitting in to the atmosphere and save 0.562 hectare of forest per year from being deforested and hence attributed towards the decrease in global warming. Generally, the geomembrane cylindrical film biodigester technology was found cheap and simple way to produce gas in the study area. Keywords: economical, feasibility, geomembrane, slurry Cite This Article: Seid Yimer, Bezabih Yimer, and Omprakash Sahu, “Biogas Production Using Geomembrane Plastic Digesters as Alternative Rural Energy Source and Soil Fertility Management.” Sustainable Energy, vol. 2, no. 1 (2014): 12-19. doi: 10.12691/rse-2-1-3.

fuels has caused serious air pollution problems, likewise the excessive consumption of fire wood results in 1. Introduction deforestation on a large scale. IUCN (1990) [5] estimated that high forests covered 16% of the land area of Ethiopia The continuous depletion of fossil fuel is sticking the in the early 1950 s, 3.6% in the early 1980 s and 2.7% in concern into the search for new energy sources to be used. 1989. Biogas digestion was introduced into developing The potential energy sources have been emerged as countries as a low-cost alternative source of energy to renewable energy resources. For a long time multifarious partially alleviate the problem of acute energy shortage for sources of renewable energy are being investigated to households, reduces deforestation and soil erosion, avoids meet the increasing energy consumption rate. To scarcity of firewood, benefits environment globally and counteract with the growing demand researchers are provides excellent fertilizer [6]. exploring the new sources. The developing countries are Biogas plants are a closed container in which anaerobic going ahead to face the shortage of available energy. fermentation of cellulose containing organic material takes Dependency from biomass such as fuel wood, charcoal, place so as to produce biogas and slurry. There are three dried cow dung cake and crop residue in Ethiopia amounts basic designs of biogas plant popular in the world. These to 95% (Benjamin, 2004). According to MOA (2000), on are the floating-drum type, fixed-dome type and bag average each rural household spends ten hours per week digester. The floating-drum and fixed-dome type biogas searching for fuel wood. Females & children are engaged plants have been introduced to Ethiopia. However, they to search fire wood for about 5-6 hours journey [1]. When became an obstacle to the rapid diffusion of biogas all forest uses are included, the deforestation rate in technology, because it takes a relatively long time (3-5 Ethiopia is around 1.1% per year [2]. According to Bech week) to build a plant, high initial cost of investment, et al. [3], the forest cover of North Wollo and Habru shortage of adequate skilled person who can undertake district is 37,183.58 hectare and 1614 hectare respectively. construction & installation of the plant and transportation According to FAO (2000) [4], the combustion of fossil problem of the prefabricated steel drum from the urban

Sustainable Energy 13 areas to the interior rural regions of Ethiopia [7]. The Number of cattle required was calculated by using the introduction of the geomembrane plastic bag digester have formula, not yet been experimented in Ethiopia. Considering the Totaldung problem of biogas technology dissemination with the (2) existing biogas plants, the study was conducted on Dung/ animal alternative biogas plants constructed using geomembrane The volume of the digester was calculated as plastic in cylindrical shape. In this regard’s an effort has been made to introduce geomembrane plastic biogas plant, Quantity of daily dung required / digester comparisons of gas and slurry yield and economic Weightof() dung+ water (3) feasibility analysis with the fixed-dome biodigester should = Densityofslurry be done and accordingly, the outcome of the study may have some contribution to set remedy to the problem. Minimum digester volume (DO) = Volume of daily charge * Retention time Actual digester volume = DO + 0.1 DO. 2. Materials and Methods The volume of gas holder was calculated as, 2.1. Description of the Study Area V= ( volume of gas production) (4) The experiment was conducted in Mersa Agricultural Total volume for geomembrane biodigester = Volume

T.V.E.T College, Ethiopia at 11°35’N latitude and of digester + Volume of gas holder. 39°38’E longitude with an altitude of 1557 m above sea Hydrostatic pressure due to the slurry acting on the level. The area is classified under moist warm climatic inner surface which exerts tensile load on the digester wall zone with mean annual rainfall of 1090 mm and with an was calculated as = depth of slurry * density of slurry. average daily temperature of 21.12°C. According to Santra [9], a ratio of 4 in length to one in diameter was used to produce much gas and quality 2.2. Geomembrane Plastic Biodigester Design fertilizer in horizontal biogas plants. Where D = diameter Parameters and L= length and accordingly the dimension of the cylindrical plastic biodigester was calculated as A. Selection of materials. = Construction materials: geomembrane plastic 0.5 mm in V DL2 /4 (5) thickness, PVC pipes. Input materials are cow dung and water. 2.3. Experimental Design and Layout B. Temperature: Mesophilic (21.4°C). Four biodigester were made from single & double C. Mixing ratio of substrate 1:1. I.e. 75 kg of cattle layered cylindrical geomembrane plastic film, constructed dung was mixed with 75 liters of water. Total dung below the ground on a trench excavated at a dimension of required per day was calculated as [8]. 7 m * 1.5 m * 0.5 m and above the ground surface on a Total dung required / day concrete block wall platform constructed at a dimension of 7 m*1.5 m*(0.75 m and 0.5 m) with a slope of 2°. One Gasproduction/ day (1) = Chinese model fixed-dome biogas plant was also another Gas/ Kgofreshdung treatment. The capacities of the digesters were 3 m3. The D. Hydraulic Retention Time (HRT). experiment duration was from Nov. 2011 to July 2012. For mesophilic temperature of Mersa locality, HRT is The treatments of the experiment were: selected as 40 days.

Figure 1. Layout of the Experimental Site

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 PSA: plastic biodigester, single layered and during the study. Amount of gas production, quantity of constructed above ground. input and output slurry, temperature of the air, pH of the  PDA: plastic biodigester double layered and fresh cow dung and digested slurry, total–N and organic constructed above ground. matter content of the substrate and the slurry. Daily  PSU plastic biodigester single layered and rainfall, temperature of the air and slurry was also constructed under ground. measured.  PDA plastic biodigester double layered and constructed underground. 2.5.1. Input to the Digesters  FDU: Fixed dome biodigester constructed The type of input material which was found feasible underground. and available in the study area was cow dung as there was The manure was collected from Mersa Agricultural dairy farm at a distance of 25 meter from the experiential TVET College dairy farm and the nearby private cattle site. Manure inputs were measured using bucket of 25 liter. shed. One bucket of cattle dung was mixed with 1 bucket of water [8]. Thus, as per the design three bucket of dung (75 2.4. Geomembrane Plastic Construction Kg) were mixed with three bucket of pure water (75 litres) Methodology so as to produce 3 m3 gas per day on May 5, 2007. After the slurry mixture has been fed in to the digester, 15 liters Materials used for the construction of geomembrane starter material prepared from cattle dung and water in 1:1 plastic biodigester were geomembrane plastic, PVC pipes, ratio and allowed to ferment for one month in a closed gate valves, GI caps, sockets, nipples, neoprene rubber barrel were added at equal amount to all five biodigester hose and biogas stoves. Construction was done with the to initiate and facilitate the fermentation process. help of electrical geomembrane welding machine and CM-43 adhesive with other mixtures. The geomembrane 2.5.2. Measurement of Gas Production plastic was cut at a dimension of 7 meter length and 4.50 The quantity of gas produced from the two models of meter width as per the design. Then, it was welded with biodigester were measured with the help of standard sized the help of electrical plastic welding machine across the biogas burner which is, certified by ISO and manufactured length and the circular part of the cylinder was fitted with by gas crafters in Bombe, India. The burner has a capacity the help of CM-43 adhesive and with other chemical of 0.45 m3 per hour and it was adjusted with the help of its mixtures. The biodigester has an inlet for entry of input air shutter until blue flame comes to burn with its materials, gas outlet for exit of produced gas and slurry maximum capacity. Time to burn was taken by stop watch outlet for disposal of fermented slurry. The digester and for consecutive hours of one day. So the daily gas gas holder was made as one unit in a cylindrical shape. production from the digester (s) is the sum total of the Table 1. Technological Parameters of the Experimental hours run by each burner and its gas consumption rate. geomembrane plastic biodigester 2.5.3. Temperature of the Air and Slurry Constants Value The temperature of the air was measured via mini-max Plastic width, m 2 thermometer in a metrological station found around the study area and the daily temperature of the slurry in the Circumference, m 4 biodigester was measured using ordinary electronic thermometer. Internal diameter m 1.24 2.5.4. Total-Solids (DM) Content Plastic length, m 6.2 The dry matter content of the substrate and spent slurry Loading rate, Kg ODM/m3/day 10.36 was measured by drying a sample at 70°C in an oven and weighing the residue on a precision electronic balance. Retention time, days 40 2.5.5. The Organic Dry Matter (ODM) Quantity of daily dung required/digester, m3 0.136 Only the organic or volatile constituents of the feed material are important for the digestion process. For this 3 Minimum digester volume, m 5.44 reason, only the organic part of the dry matter content was considered and this was analyzed in the laboratory. Actual digester volume, m3 5.984 2.5.6. PH of the Fresh Cow Dung and Digested Slurry Daily volume of gas production, m3 3 The pH of the fresh cow dung and fermented manure Total volume of geomembrane plastic biodigester 7.484 was measured by pH-meter in the laboratory.

Hydrostatic pressure due to the slurry, Kg/m2 1023 2.5.7. Quality of Output Slurry The compositions of digested slurry produced by 2.5. Data Collection Procedures anaerobic fermentation in the two models of biogas plants were determined in the laboratory. Five digested slurry Different data which were pertinent to the study samples from all five biogas plant and two fresh manure objectives were collected following standard procedures. samples from the cow dung fed to the two types of biogas The following variables were measured and analyzed plant were taken by random sampling technique.

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According to AOAC (1990) [10], total-N was determined their specific gas production. Specific gas production was by Foss-Tecator Kjeldahl procedures after taking 1 gram determined by dividing the daily volume of gas measured of manure sample and digested with sulpheric acid and by the amount of cow dung loaded into the plant. salicylic acid and estimated by Kjeldahl method. The process of digestion took about three and half hours in the laboratory. 3. Results and Discussion Organic matter content was determined by the use of Toffle furnace by ash method. 10 gram of well mixed 3.1. Biogas Production manure in dry nickel crucible or silica basin was weighted Gas was burnt and measured after gas has completely and was put in a low flame or hot plate till the organic produced within the designed HRT of 40 days with the matter begins to burn. The crucible in a muffle furnace help of calibrated biogas burner and stop watch (Figure 2). was placed at about 550°C for 8 hours. The crucible with As can be seen in Table 2, gas production as the a grayish white ash formed was removed from the furnace proportion of biodigester liquid volume was higher for a cool in a desicator and weigh. Therefore, the residue single layered and above ground plastic biodigester than represents the ash and the loss in weight represents the others and very less amount of gas was measured from the moisture and organic matter [11]. Fresh cow dung samples fixed-dome biodigester. This was because more sun light were taken on April, 2011 and fermented slurry samples temperature (27.65°C-32.7°C) was absorbed in a black were taken after gas was fully generated and measured i.e. geomembrane plastic sheeting digester than reinforced on June, 2011 from five biodigester and two cattle sheds. concrete fixed-dome biodigester (Figure 3). Temperature 2.5.8. The Efficiency of Biodigester is very important factor which positively or negatively affects the activity of microorganisms in the production of It was evaluated by comparing its gas and slurry biogas. production with the amount of substrate fed in relation to the volume of the digester and compared by calculating

Figure 2. Burning and Measuring of Biogas with a Biogas Burner after Gas Generation

3200 y = 106x - 421.52 2 3000 R = 0.9418

2800

2600

2400

2200 Amount of gas litre/day/unit gas of Amount

2000 25 27 29 31 33 35 Temperature of the slurry,Oc

Figure 3. The Relationship between Temperature of the Slurry and amount of Gas Production for the Plastic and Fixed-Dome Biodigester

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Table 2. Total Values for Biogas Production (9 Am to 4 Pm) in Biodigester with Different Types, Layers and Location of Installation

BIODIGESTER TYPE Biogas Production PDA PSA PDU PSU FBU Average for plastic digesters

Hours recorded during burning of gas in a burner /plant 6.33 6.75 5.75 5.50 5.25 6.08

Liters/day/plant 2850 3037.5 2587.5 2475 2362.5 2737.5

Specific gas production per day (m3/kg) 0.0380 0.0405 0.0345 0.033 0.0315 0.0365

Average daily slurry temperature °C 29.99 32.7 29.18 27.65 25.96 29.88

PDA: plastic biodigester, double layered and constructed above ground PSA: plastic Biodigester, single layered and constructed above ground PDU: plastic biodigester, double layered and constructed underground PSU: plastic biodigester, double layered and constructed underground FBU: fixed-dome biodigester constructed underground. of the fermented material in the biodigester were in the 3.2. Temperature of the Air and Slurry range between 25.96°C and 32.7°C as described in (Table The average atmospheric temperature during 3), which is greater than the critical 15°C. Thus, gas was fermentation time between May 5, 2007 and June 14, completely produced within the designed HRT value of 40 2 2007 was 24.55°C. According to Grewal et al. [8], days. As it is mentioned in Figure 3, R is 0.945. Thus, temperature is one of the factor affecting the growth rate 94.5% of the variability in the amount of gas produced by of micro-organisms involved in the production of biogas the biodigester was due to the variation in the slurry and effective anaerobic fermentation is carried out in temperature. mesophilic temperatures averaging between 24°C and The average temperature of the slurry measured in the 45°C. The local average atmospheric temperature of geomembrane plastic biodigester exceed by 1.687°C to Mersa was 21.12°C according to 12 years of data which 6.737°C of the slurry in fixed-dome biodigester (Table 3). was very suitable for biogas production. Thus, geomembrane bag was observed to have the best According to Hu Qichun (1991) [12], a biogas plant advantage of heating the digester contents easily and 3 3 could perform satisfactorily only where mean annual produce higher amount of gas (0.12-0.68 m of gas per m temperatures are around 20°C or above or when the of digester per day) than fixed-dome biodigester made average daily temperature is at least 18°C. With the range from reinforced concrete (Table 3). Since its walls are thin of 20-28°C mean temperature, gas production increases and black in color, it can be heated quickly with an over proportionally. If the temperature of the biomass is external heat source, such as the sun radiation of the same below 15°C, gas production will be so slow that the biogas degree in the study area. Similar results were reported by plant is no longer economically feasible. So, the Bui Xuan and Preston [13] that found average temperature recorded in the study area was very suitable temperatures in bag digesters, compared with dome types, 3 for normal fermentation and higher amount of gas was are 2°C-7°C higher in the bag (0.235-0.61 m of gas per 3 produced by mesophilic bacteria. The average temperature m of digester per day).

Table 3. Comparison of Average Slurry Temperature, °C and amount of Gas Produced, m3/Day of the Biodigester Types of Average Slurry temperature, °C of Amount of gas produce, GPBD, °C–FBU GPBD, m3/day–FBU, 2.36 GPBD GPBD m3/day ( 25.963 °C ) m3/day PSA 32.700 3.04 6.737 0.68 PDA 29.988 2.85 4.025 0.49 PDU 29.175 2.59 3.212 0.23 PSU 27.650 2.48 1.687 0.12 GPBD: geomembrane plastic biodigester reported by UNESCO (1982) [14], and states that during 3.3. Characteristics of Bio-Digested Slurry digestion, about 20% of the total slurry is volatilized. Thus, (Effluent) and the Influent geomembrane plastic biogas plant produced higher amount of slurry than fixed-dome biogas plant of the same Organic substances passed through biogas plants not capacity and using equal amount of input material. So, only produce a source of energy, but also a large quantity with the use of geomembrane plastic biodigester, the of digested slurry, which provides excellent organic farmers could get 45 tones of fermented slurry which fertilizer. The net weight of slurry discharged daily from could be used to apply on the farmland as organic the geomembrane plastic and fixed-dome biodigester were fertilizer. Thus, by conversion of cow-dung in to a more 123 Kg and 112.5 Kg respectively. Thus, the annual slurry convenient and high-value fertilizer (biogas slurry), output was 44895 Kg (45 tons i.e. 18% of the total organic matter is readily available for agricultural substrate was lost) and 41062.5 Kg (41 tons i.e. 25% of purposes, thus protecting soils from depletion and erosion. the total substrate was lost), which was produced from Therefore, the farmers should be advised to use 27375 kg of fresh dung fed to the digester annually, which geomembrane plastic biogas plant so as to utilize the dung in turn is equivalent to 13468.5 kg of dried dung cakes. for dual purposes such as the produced gas as fuel for Likewise, the loss in the amount of slurry have been cooking and the remaining large quantity of slurry as

Sustainable Energy 17 organic fertilizer which helps to increase crop production up by many plants and in a number of applications, slurry by preventing it from burning in the form of dry dung from biogas plants is even superior to fresh dung cake and ashing down the good manure creating especially when the slurry is spread directly on fields with unhygienic conditions in the kitchen. a permanently high nitrogen demand (e.g. fodder grasses) or when using slurry compost to improve the structure of 3.4. Characteristics of Total-N in the Slurry the soil. and Influent Processing evidence suggests, however, that slurry is much more effective than dung when applied as fertilizer, The average total nitrogen of the substrate and digested French (1979) [15], discusses that slurry is 13 % more manure of the geomembrane plastic and fixed-dome effective than dung, and Van Buren (1974) [16], reported biodigester were 0.37%, 1.13% and 1.15% respectively as that the ammonia content of organic fertilizer fermented described in Table 4. Similarly, Grewal et al. [8], reported for 30 days in a pit in china increased by 19.3%. that the total-N content of fresh dung as 0.242% which is As can be seen in Table 4, the single layered 34.6% less than the result of this study. Thus, fermented geomembrane plastic biodigester constructed above slurry contained larger nitrogen content than fresh cow ground produced higher amount of total nitrogen than dung in both models of the biodigester because in an air others. This was due to higher amount of slurry tight biogas digester more organic acid such as acetic acid, temperature (Table 3) absorbed in the single layered above prop ionic acid, butyric acid, ethanol and acetone was ground biodigester activated anaerobic micro-organisms produced doting anaerobic fermentation of soluble simple to convert more simple organic substances of the substrate organic substances which helps to absorb and fix ammonia in to simple organic acid so as to fix more ammonia. and minimize the loss of nitrogen thus conserving the Therefore, total-N was affected by material and position fertility of the manure. So, the higher quantity of nitrogen of biodigester construction. Mersa ATVET dairy farm was converted in to the useful nitrate and ammonia which fermented cow manure contains 1.0164%-1.2026% N and is the most important ingredient for plant growth. 50-75% Organic matter. Thus, the annual output of slurry There was also a greater conversion of organic substrate equivalent to 13468.5 kg of dry dung cake converted to to nitrogen during 40 days of the experiment because 152.2 kg of N and 8127 kg of organic matter on the microbial anaerobic degradation was facilitated with average. In the slurry which has higher total nitrogen higher temperature. content there is higher proportion of ammonium which According to Hu Qichun [12] (1991), during anaerobic constitutes the more valuable form of nitrogen for plant fermentation, part of the total nitrogen is mineralized to nutrition [17]. ammonium and nitrate. Thus, it can be more rapidly taken

Table 4. Effect of Material & Position of Biodigester Construction on the Composition of the Effluent Components of the effluent and influent Biodigester types and fresh cow dung fed to the biodigester PSU PDU FBU PSA PDA IFB IPB Average(For Plastic digesters) Organic matter, % 50 50 66.7 75 60 80 85.7 58.75 Organic matter (Kg) 37.5 37.5 50.025 56.25 45 60 64.28 44.06 Organic carbon, % 29 29 38.686 43.5 34.8 46.4 49.706 34.08 Total N, % 1.148 1.1368 1.1536 1.2026 1.0164 0.3626 0.3822 1.126 Total N, Mg/kg 11480 11368 11536 12026 10164 3626 3822 11259.5 Dry matter, % 0.2 0.2 0.5 0.6 0.4 0.7 1.0 0.35 pH 6.8 6.9 7.2 7.1 7.6 6.8 7.0 7.1 IFB: influent of fixed-dome biodigester, IPB: influent of plastic biodigester pH: hydrogen ion concentration. [8], almost all plant nutrients are retained in the digested 3.5. Characteristics of Organic Matter in the slurry in such finely divided state that, it mixes up with Slurry and Substrate the soil quickly and thoroughly, and the soil bacterial The application of digested slurry to crop serves as a activity increases substantially. Thus the application of dual purpose; a soil conditioner as well as a source of digested slurry gives better yields for all crops as plant nutrients. According to Table 4, the effluent in PSA compared to farmyard manure (FYM) made from the showed larger organic matter content than the effluent in same quantity of cattle dung. The digested slurry in this FBU. This could be because the amount of organic matter study was thin and used directly to the crops through the in the influent of plastic biodigester (IPB) was higher than irrigation channels and by direct splashing on the farmland. that in the influent of fixed-dome biodigester (IFB). Moreover, it was also put in a fish pond of Mersa Higher value of organic matter was recorded from fresh agricultural T.V.E.T. College and fishes were nourished. cow dung than the slurry (Table 4). Even if higher value Thus, it was proved that, the waste that comes out of the was recorded for fresh manure, the use of excreta of dairy digestion process as slurry was very useful both as feed cattle in biogas plants was found to be better than the and organic fertilizer. Studies by Sokoine Agricultural farmyard manure in several ways. A part of nitrogen University in Tanzania have shown that slurry from biogas which is ammonia, found in the slurry becomes available improves productivity of land and maintains soil quality to the plants. The ability of the wet digested slurry to that can support crop production over a long period of aggregate soil particles immediately after application is time [18]. also very unique. The digested manure was available in 40 days as compared to 4-6 months taken in the usual method 3.6. Characteristics of pH of Fermented of composting in a manure pit. According to Grewal et al. Slurry

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It is generally recommended that the pH inside the to Woldya market in 2006/2007 ranged from EB 5.00 to biodigester should be above seven for normal EB 8.00 per 50 kg sack, averaging EB 0.50 per kg of dried fermentation and maximum gas production [8]. In the dung. present study, the pH of the fermented slurry and fresh Water: This was valued according to the price charged cow-dung was 6.8-7.6 and (Table 4) with a hydraulic by the water authority of Habru District, Mersa town. I.E. retention time of 40 days. Thus, the condition inside the EB 2.00 per 5 m3. digester was very comfortable for anaerobic micro- Labor: The Labor used to collect water and spread organisms to accomplish normal fermentation, higher gas slurry was valued at EB 15 day-1. production and fertile manure. 3.8.2. Market Price of Outputs 3.7. Efficiency of the Biodigester Biogas: The biogas produced by the digestor was valued at the market value of fire wood or dried cow dung The average specific gas production of the single cakes, which it replaces. The observed price of fire wood layered above ground geomembrane plastic biodigester in Mersa was between EB 15 and EB 20 per bundle was greater than others (Table 2). That was due to higher weighing 15-20 kg. It was estimated that firewood amount of sun light was absorbed by the digester (32.7°C) averages EB 1.00 kg-1. that used to facilitate the activity of micro-organisms and Slurry: Output digested slurry was valued at the official increase the amount of gas production per weight of the price of DAP (diammonium phosphate). N and P contents substrate (Figure 2). Thus, the construction of single of DAP roughly approximated the proportions of these layered aboveground biodigester is better than other nutrients in dried cow dung. According to Senait Seyoum biodigester as efficiency of conversion of substrate [19], 1 ton of DAP is roughly equivalent to 16 ton of dry organic matter to biogas was higher. -1 manure. The 2006/2007 price of DAP was EB 1450 tone , thus each ton of dry cow dung which has less nutrient 3.8. Economic Evaluations content than the fermented slurry is worth EB 90.62. In order to compute the value of biogas in terms of the Therefore, the cost of 1 ton of fermented slurry was value of traditional fuels saved by utilizing biogas, it was assumed to be EB 90.62 to the minimum. necessary to estimate the amount of dried dung cakes or Three important technical assumptions were made with fire wood needed to produce an equivalent amount of respect to gas production and use, and the efficient use of energy. Thus, according to UNISCO [14], Van Buren [16], inputs and their conversion in the analysis. used in this study assume that 1 m3 biogas substitutes for Table 6. Summary of total costs and total benefits of geomembrane 3.47 kg of fire wood, 12.3 kg of dry dung fuel and 0.62 plastic and fixed-dome biogas plants liter of kerosene oil. When used as fuel When used as Biogas Total wood manure Economic evaluation was also done for all biodigester No using cost-benefit analysis which is the most commonly Models Benefit Total Net Total Net employed method used by many extension officers. It was Cost Benefit Cost Benefit 1 PSA 7925.05 4427.90 3498.05 5035.45 2889.6 able to evaluate the relative advantage of a geomembrane 2 PDA 7687.57 4604.69 3082.88 5222.25 2465.32 plastic biogas plant investment as compared to fixed dome 3 PSU 7212.61 4127.78 3084.83 5562.47 1650.14 biodigester on the basis of the anticipated minimum 4 PDU 7355.10 4191.63 3164.10 5229.25 2125.85 interest rate and economic life for the alternative designs. 5 FBU 6710.14 5662.96 1047.18 6389.32 320.82 A discount rate of 18 % has been applied throughout the 1. The average daily gas production for a year from the geomembrane plastic and fixed-dome biodigester analysis according to Amhara regional state credit and 3 3 saving institution (2007). was assumed to be 2.74 m and 2.36 m as to the measurement taken once in drier months, but this 3.8.1. Market Price of Inputs could vary considerably with daily ambient temperature fluctuations in a year. Dung: According to Senait Seyoum [19] (2007), the 2. In the analysis it was assumed that all gas produced cost of dung was estimated in terms of. would be used, and it would have the same use as the dry dung or wood replaced by biogas. Table 5. Summary of Market Value of Inputs and Outputs Used in 3. The amount of slurry produced daily from the the Analysis Inputs Market value (price in EB) geomembrane plastic and fixed-dome biodigester Dung (EB/kg) 0.50 was 123 tons and 112.5 tones. Water (EB/5 m3) 2.00 As per the design, the total quantity of fresh dung Labor (EB/day) 15.00 required for 3 m3 size biogas plants in one year was 27375 Outputs Biogas (EB/litre) 3.47 kg or 27.375 tons but the quantity of fermented slurry Slurry (EB/tone) 90.62 collected after digestion and gas measurement was 45,000 EB: Ethiopian Birr (1$ = 18 Birr) kg and 41,000 kg in the geomembrane plastic and fixed-  Dung’s value as fertilizer determined by the cost of dome biodigester respectively. Thus, approximately 10 an equivalent amount of commercial fertilizer, or. tons and 14 tons of digested slurry (18% and 25% of the  The market value of dung cakes, if dung is sold. input mixture) were lost during fermentation from the total  1 ton of DAP is 16 tone of dry manure. of 55 tons of slurry mixture available in the geomembrane Price of 1 tone of DAP according to 2006/2007 year plastic and fixed-dome digesters respectively. The loss in was 1450 ET Birr. This was determined from a receipt the weight of the slurry was due to the loss of solids voucher issued to Mersa agricultural T.V.E.T College during fermentation. Similarly, UNESCO (1982), states purchasing office. The price of dried cow dung according that, during digestion, about 20% of the total slurry is

Sustainable Energy 19 volatilized. Thus, about 80% of the manure is collected [2] Wikipedia, 2006. Retrieved on Sep 3, 2007 available at from fresh dung. According to Grewal et al. [18], the loss http://en.wikipedia.org/wiki/Ethiopia. of solids in the biogas plant rarely exceeds 27 percent [3] Bech, N., Waveren and Evan, 2002. Environmental Support Project (ESP), Component 2: Environmental Assessment and even when maximum gas is generated. Sustainable Land Use plan for North Wollo. [4] FAO (Food and Agricultural Organization of the United Nations), 3.8.3. Cost-Benefit Analysis of Biogas Plants (2000). Statistical data base. FAO, Rome, Italy. [5] IUCN, 1990. Ethiopian National Conservation Strategy. Phase 1 Therefore, the net benefit of geomembrane plastic Report. Based on the work of M.Stahl and A. Wood. IUCN, Gland, biogas plants is greater than fixed-dome biogas plant and Switzerland Kristoferson LA and Bokhalders, 1991. Renewable in particular the net benefit gained from PSA is greater Energy Technologies: Their application in developing countries. than others. Thus, the use of geomembrane plastic Intermediate Technology Publications, London, pp 112-117. biodigester is profitable than fixed-dome biodigester. [6] Vandana S, 2004. Alternative Energy. APH Publishing Corporation. New Delhi-11002. [7] Yacob Mulugeta, 2000. ‘Renewable Energy Technologies and 4. Conclusion Implementation Mechanisms for Ethiopia’, Energy Sources, 2 (1). [8] N.S.Grewal et al., 2000. Hand Book of Biogas Technology (A Generally, gas production and total nitrogen content as practical hand book). Punjab Agricultural University, Ludhiana. the proportion of biodigester liquid volume was higher for India. [9] S.C.Santra, 2001. Environnemental Science. New central book a single layered and above ground geomembrane plastic agency (p). l td. India. biodigester than the fixed-dome and other plastic [10] AoAC, 1990. In: Helrick (Ed), official Methods of Analysis. biodigester. So the construction and use of single layered Association of Official Analytical Chemists 15th Edition. geomembrane plastic biodigester above the ground surface Arilington. pp. 1230. Retrieved on December 12, 2007 available at is preferable than other models and locations of www.mekarn.org/msc 2003-05/thesis 05/tram-p, pdf. [11] Tekalign Tadesse, L.Haque and E.A.Aduayi. 1991. International installations of the biodigester. Fermented slurry contains Livestock center for Africa. Addis Ababa, Ethiopia. larger nitrogen content than fresh cow dung in both [12] Hu Qichun, 1991. Systematical Study on Biogas Technology models of biodigester. Thus fermented slurry has high Application in Xindu Rural Area, China. Asian Institute of fertilizer value in increasing the fertility of the soil than Technology, Bangkok/Thai. fresh cow dung. The geomembrane plastic biodigester [13] Benjamin Jargstorf, 2004. Renewable energy and Development Brochure to accompany the mobile exhibition on Renewable gave higher net benefit than the fixed-dome biodigester. energy in Ethiopia. Addis Ababa. Thus, the geomembrane plastic biodigester is the cheapest [14] UNESCO (United Nations Educational, Scientific and Cultural model that an individual farmer could invest and acquire a Organization), 1982. Consolidation of information. Pilot edition. better profit than the fixed-dome biodigester. General information programme and universal system for Considering the long-term benefit of plastic film information in science and Technology, UNESCO, Paris. [15] French D, 1979. The economics of renewable energy systems for biodigester technology both economically and developing countries. environmentally, it is recommended to introduce the [16] Van Buren A (Ed), 1974. A Chinese Biogas Manual: Popularizing single layered above ground geomembrane plastic biogas technology in the countryside. Intermediate Technology plant to be used for the beneficiaries regardless of its Publications, London. [17] Werner Kossmann, Uta Ponitz, 2007. Biogas Digest Volume 1. higher net benefit, higher gas and fermented slurry Information and Advisory Service on Appropriate Technology production, simple construction and maintenance via (ISAT). Retrieved on December 13, 2007 from extension education to promote its penetration and www.gtz.de/de/documente/en-biogas_volume 1. pdf. diffusion into rural areas. However, greater safety [18] SURUDE (Foundation for sustainable Rural Development), 2002. precaution during operation and usage of the plant and Promotion of low cost biogas technology to resource poor farmers in Tanzania. Retrived on July 8, 2007 from http://www.equator protection from damaging agents such as sharpened initiative.net. objects and rats is essential. [19] Senait Seyoum, 2007. The economics of a biogas digester. Retrieved on Sep 15, 2007 from http://www.ilri.org/infoserv/webpub/Fulldocs/Bulletin30/economi. References htm. [1] MOA (Ministry of Agriculture), 2000. Agro ecological Zones of Ethiopia, Natural Resource Management and Regulatory Department, Addis Ababa.